netdev - Re: [PATCH v6 net-next 1/7] netlink: Add a macro to set policy message with format string

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Message-ID: <20230322114041.71df75d1@kernel.org>
Date:   Wed, 22 Mar 2023 11:40:41 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Shay Agroskin <shayagr@...zon.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
        "Woodhouse, David" <dwmw@...zon.com>,
        "Machulsky, Zorik" <zorik@...zon.com>,
        "Matushevsky, Alexander" <matua@...zon.com>,
        Saeed Bshara <saeedb@...zon.com>,
        "Wilson, Matt" <msw@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "Bshara, Nafea" <nafea@...zon.com>,
        "Belgazal, Netanel" <netanel@...zon.com>,
        "Saidi, Ali" <alisaidi@...zon.com>,
        "Herrenschmidt, Benjamin" <benh@...zon.com>,
        "Kiyanovski, Arthur" <akiyano@...zon.com>,
        "Dagan, Noam" <ndagan@...zon.com>,
        "Arinzon, David" <darinzon@...zon.com>,
        "Itzko, Shahar" <itzko@...zon.com>,
        "Abboud, Osama" <osamaabb@...zon.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Andrew Lunn <andrew@...n.ch>,
        Guangbin Huang <huangguangbin2@...wei.com>,
        Jie Wang <wangjie125@...wei.com>,
        Johannes Berg <johannes@...solutions.net>,
        Edward Cree <ecree.xilinx@...il.com>,
        Florian Westphal <fw@...len.de>
Subject: Re: [PATCH v6 net-next 1/7] netlink: Add a macro to set policy
 message with format string

On Wed, 22 Mar 2023 14:39:49 +0200 Shay Agroskin wrote:
> > You should use '__extack' even above, to avoid multiple 
> > evaluation of
> > the 'extack' expression.  
> 
> I've got to admit that I don't understand the cost of such 
> evaluations. I thought that it was added to help readers of the 
> source code to understand what is the type of this attribute and 
> have a better warning message when the wrong variable is passed 
> (kind of typing in Python which isn't strictly needed).
> What cost is there for casting a pointer ?

It's not about the cost, the macros are unfolded by the preprocessor,
in the unlikely case someone passes extack++ as an argument using it
twice inside the body of the macro will increment the value twice.

#define MACRO(arg) function_bla(arg, arg) // use arg twice

int a = 1;
MACRO(a++);
print(a); // should print 2, will print 3